It is well known to spray-apply fireproofing slurries to metal structural members and other building surfaces to provide a heat resistant coating thereon. Spray-applied, hydratable, cementitious fireproofing compositions are disclosed, for example, in U.S. Pat. No. 4,751,024 of Shu et al.; and U.S. Pat. No. 4,904,503 of Hilton et al.; U.S. Pat. No. 5,034,160 of Kindt et al.; and U.S. Pat. No. 5,340,612 of Perito, which are owned by the common assignee hereof and incorporated herein by reference. These fireproofing compositions teach the use of lightweight aggregates, such as shredded polystyrene particles, to obtain concentrations favorable for pumping the fireproofing composition through hoses, sometimes to great heights, to nozzles from which the fireproofing is sprayed onto a substrate surface using compressed air. Moreover, the fireproofing compositions provide non-segregating, uniform slurries displaying desired levels of adhesion to steel substrates and excellent fire protection. The density of the slurry compositions containing concentrations of lightweight aggregate is sufficiently low after application to ensure that the coating stays in place before and after setting. Thus, "fall off" of the applied composition due to inadequate adherence of mechanical and structural disturbance is minimized or eliminated. One of the unavoidable features of spray-applying cementitious fireproofing, however, is the need for achieving certain coating thicknesses. Such coating thicknesses are needed to meet fireproofing standards, and are attained, for the most part, through successive spray passes over the substrate surface. For example, U.S. Pat. No. 5,340,612 of Perito disclosed a sprayable fireproofing composition that provided a quick setting time such that multiple coatings of the slurry could be applied within the same working day. Nevertheless, there remains a concern that using multiple coatings to achieve minimal thicknesses sufficient for attaining fireproofing protection is time-consuming and labor-intensive.
The present invention pertains to the surprising discovery that the fireproofing performance of conventional spray-applicable cementitious fireproofing may be considerably enhanced using a secondary, overlying layer formed from an aqueous dispersion of delaminated vermiculite platelets. In other words, the present inventors envision that it is possible to use a relatively thinner coating of cementitious fireproofing without substantially sacrificing fireproofing efficacy. The ability to apply the fireproofing in a relatively thinner coating will conserve labor and energy, and expenses and time at the application site.
The present invention also does not need to rely upon lightweight or heavy aggregate, which may be optional, and thus provides the ability to use even thinner wet pass coatings that afford a savings in time and labor.